1. Field of the Invention
The present invention relates to a focal length switch-over camera cable of e.g. standard photography and telephotography, or the standard photography and a wide-angle photography, and more particularly to a camera of the above-mentioned type having control means capable of effecting a switch-over of the focal length of, and a focus adjustment, of a photographic optical system in a serial and continuous manner.
2. Description of the Prior Art
There is a known focal length switch-over camera of the above-mentioned type having control means constituted by a screw member extending in a direction of optical axis and a mount plate of the photographic optical system engaging the screw member. In the case of this conventional camera, the focal length switch-over operation and the focus adjustment operation in the respective focal length conditions are carried out by drivably rotating the screw member by means of a motor or the like thereby moving the photographic optical system mounted on the mount plate in the direction of the optical axis. See e.g. a Japanese patent application published under SHOWA No. 60-122931.
However, this camera has problems to be described hereinafter.
The amount of movement of the photographic optical system for switching over the focal length thereof is generally larger than that of the same system for effecting a focus adjustment in the respective focal length conditions. Also, this movement amount of the photographic optical system for effecting a focus adjustment in the respective focal length conditions increases with an increase in the focal length.
Therefore, in the conventional control means, the screw member is employed for moving and fixedly positioning the photographic optical system. This screw member has a thread pitch uniform over its whole length and thus the movement amount of the photographic optical system relative to the rotational amount of the screw member is always the same. Therefore, unless the motor is of the variable speed type, thereby complicating the control of the same, and if the thread pitch of the screw member is small for effecting with a high precision the focus adjustment operation in the shorter focal length condition in which the movement amount of the photographic optical system is small, the focus adjustment operation and the focal length switch-over operation in the longer focal length condition takes a long time. On the other hand, if the thread pitch of the screw member is large for effecting more speedly the focal length switch-over operation and the focus adjustment operation in the longer focal length condition in which the movement amount of the photographic optical system is large, it becomes difficult to effect with a high precision the focus adjustment operation in the shorter focal length condition.
Further, generally in the case of the camera of the above type, it is necessary to detect a delivery position of the photographic optical system in effecting a focus adjustment operation and thus there is provided a detecting device for detecting this position of the photographic optical system. A conventional position detecting device of this kind includes a position detecting member attached to the photographic optical system and a sensor for detecting a passage or a contact of the member. In this case, it is necessary to reserve space specially for disposing the member to be attached to the photographic optical system and for the sensor, whereby it becomes difficult to form the camera compact.
There is also a known focal length switch-over mechanism for switching over a focal length by sliding along an optical axis a fore-element of the photographic optical system between a position projected from a camera body and a position receded from the same.
This focal length switch-over mechanism includes a screw member extending in the direction of the optical axis and a mount plate for the optical system. In operation, the focal length switch-over operation and the focus adjustment operation in the respective focal length conditions are carried out by drivably rotating the screw member by means of a motor or the like thereby moving the optical system mounted on the plate in the direction of the optical axis.
However, in the case of a camera having such focal length switch-over mechanism, the movement amount of the fore-element for switching over the focal length is relatively large. Thus, a member, e.g. a movable lens barrel, for supporting the fore-element at its projected position projects considerably from the camera body. Accordingly, this projecting portion of the camera is very likely to be subjected to external shocks, and especially when the camera is placed with the fore-element being positioned downwardly, the total weight of the camera body is applied to this projecting portion. Moreover, since the screw member of the focal length switch-over mechanism and the mount plate for the fore-element are constantly coupled with each other, if, as described above, any shocks or external forces are applied to the portion considerably projected from the camera body, all the shocks and forces are directly transmitted to the focal length switch-over mechanism thereby often damaging the same.